Optical disks have been used extensively as information storage media that can store a huge amount of information. And as technologies have been marching on, optical disks with even bigger storage capacities have been developed one after another.
The optical disk that became popular earlier than any other type of optical disk was a compact disc (CD), which was then followed by digital versatile discs (DVDs). A DVD can store information at a storage density that is approximately six times as high as a CD's. Thus, a large amount of information can be stored on a single DVD. That is why DVDs have been used to store video that has a particularly huge amount of information. Meanwhile, next-generation optical disks that can store information at even higher densities, including Blu-ray Discs (BDs) and HD-DVDs, have been developed recently and are lately used by more and more general consumers to store high-definition video.
To increase the capacity of an optical disk, the storage density thereof needs to be increased, which can be done by decreasing the spot size of a light beam for use to perform read and write operations. And to decrease the spot size of a light beam, the wavelength of a laser beam as a light source needs to be shortened and the numerical aperture (NA) of an objective lens that forms the light beam spot needs to be increased. For example, a light source with a wavelength of 660 nm and an objective lens with a numerical aperture (NA) of 0.6 are used in combination for DVDs. Furthermore, next-generation optical disks, on which information can be stored five times as densely as on ongoing DVDs, are realized by using a blue laser beam with a wavelength of 405 nm and an objective lens with an NA of 0.85.
As the number of types of optical disks commonly used increases, the compatibility of an optical disk drive becomes more and more important. To come in handy for its users, an optical disk drive is preferably compatible with multiple types of optical disks. Specifically, an optical disk drive compatible with next-generation optical disks is preferably able to read and write from/to CDs and DVDs. In that case, however, it is difficult to make the working distance of an objective lens as long as the one defined for DVDs and CDs while increasing the numerical aperture of the objective lens to 0.85. For that reason, an optical information processor that can perform a read/write operation on next-generation optical disks preferably includes an objective lens for next-generation optical disks and at least one more objective lens to read and write from/to CDs and DVDs.
An objective lens is driven in a focusing direction and in a tracking direction by an objective lens actuator that includes a magnetic circuit. That is to say, the objective lens is controlled to maintain a predetermined gap with respect to an optical disk in the focusing direction and to follow the center of the tracks in the tracking direction.
For that reason, in an optical information processor designed to process multiple types of optical disks with mutually different storage densities, a plurality of objective lenses needs to be provided for a mover so as to be movable both in the focusing direction and in the tracking direction. Patent Document No. 1 discloses an example of such an optical information processor. As shown in FIG. 12, in the optical information processor of Patent Document No. 1, the light beam 61 emitted from a first light source (not shown) is transformed by a collimator lens 62 into a substantially parallel light beam, which is then refracted by a planar vertical reflecting mirror 63 such that its optical axis intersects with a high storage density optical disk 65 at right angles. A first objective lens 65 converges the light beam 61 onto a storage layer of the optical disk 65.
Meanwhile, the light beam 66 emitted from a second light source (not shown) is transformed by a collimator lens 67 into a substantially parallel light beam, which is then refracted by a planar vertical reflecting mirror 68 such that its optical axis intersects with a low storage density optical disk 70 at right angles. A second objective lens 69 converges the light beam 66 onto a storage layer of the optical disk 70.
An objective lens actuator 71 can move the first objective lens 64, which is fixed on a mover (not shown), both in the focusing direction F, which intersects with the storage layer of the high storage density optical disk 65 at right angles, and in the tracking direction T on the optical disk 65. Likewise, an objective lens actuator 72 can move the second objective lens 69, which is fixed on a mover (not shown), both in the focusing direction F, which intersects with the storage layer of the low storage density optical disk 70 at right angles, and in the tracking direction T on the optical disk 70. In this manner, in the optical information processor disclosed in Patent Document No. 1, the two objective lenses are driven by two separate objective lens actuators.
On the other hand, Patent Document No. 2 discloses an optical information processor in which two objective lenses are held by the same mover and are driven in the focusing and tracking directions by a single objective lens actuator.
Patent Document No. 3 discloses another example of such an optical information processor in which two objective lenses are held by the same mover and are driven in the focusing and tracking directions by a single objective lens actuator. Such an optical information processor includes three light sources to process three types of optical disks with mutually different storage densities as shown in FIG. 13. Specifically, the optical information processor includes a semiconductor laser 73, a DVD module 85 including a semiconductor laser, and a CD module 83 also including a semiconductor laser.
The semiconductor laser 73 emits a laser beam with a wavelength of 408 nm and is used to read and write information from/on a high density optical disk. The light beam emitted from the semiconductor laser 73 is transmitted through a collimator lens 74 and then incident on a half mirror 75. Part of the light that has been incident on the half mirror 75 then enters a monitor photodiode 78, but most of the light then enters a vertical reflecting mirror 88, which reflects the incoming light toward an objective lens 80. The light reflected from the optical disk initially follows the same path in the opposite direction. But after having been incident on the half mirror 75, the light is transmitted through a cylindrical lens 77 and then received at a photodetector 76.
The DVD module 85 includes a red semiconductor laser that emits a laser beam with a wavelength of 658 nm and a photodetector, and is used to read and write information from/on DVDs. The light beam emitted from the semiconductor laser of the DVD module 85 is transmitted through a DVD collimator lens 86 and a polarization beam splitter 87 and then incident on the vertical reflecting mirror 88, which reflects the incoming light toward an objective lens 81. The light reflected from the optical disk initially follows the same path in the opposite direction and then enters the photodetector of the DVD module 85.
The CD module 83 includes a red semiconductor laser that emits a laser beam with a wavelength of 785 nm and a photodetector, and is used to read and write information from/on CDs. The light beam emitted from the semiconductor laser of the CD module 83 is transmitted through a CD collimator lens 84 and the polarization beam splitter 87 and then incident on the vertical reflecting mirror 88, which reflects the incoming light toward the objective lens 81. The light reflected from the optical disk initially follows the same path in the opposite direction and then enters the photodetector of the CD module 83.
The objective lenses 80 and 81 are held by a holder 79 that operates as a mover. The holder 79 is supported by wires 82 so as to be movable in the focusing direction and in the tracking direction.    Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2002-208173    Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 11-120587    Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2005-293686